There's nothing like going out on a high. Due in Auckland this week for the Writers & Readers Festival, Eoin Colfer will conclude the adventures of teenage criminal mastermind Artemis Fowl in July with eighth instalment The Last Guardian, which he promises will bring the best-selling young adult fantasy series to a fitting finale.

"I never wanted it to go on indefinitely," he says. "Initially I thought that three books would be enough but I just kept having more ideas. But I was at the point where I would have to sign to do another couple of books. Obviously, the money would have been good but that's not a good reason to write a book. I really wanted to end with a number one book and I felt that if I dragged it on much longer, sales would inevitably start to slide, especially without a movie behind us, as you can't stay at that level forever."

Colfer cites the example of Ricky Gervais, who ended The Office on his own terms. "He said he wanted to finish it while they were on top and I really understand that," he says.

"I wanted to leave it as a body of work I'm happy with. I wanted to finish it with a really strong story that people would like as much as the other books and not just buy out of loyalty and then be a little disappointed."

A former primary school teacher who had previously penned a couple of books for younger readers, the 47-year-old admits the series has grown since the inaugural volume, Artemis Fowl, was published in 2001.

"It's become rather grander in scale," he says. "The first book was like a morality tale and was very much an adventure story, so the characters weren't really developed. It had a very simple structure and it moved along at a pace, which was one of the points of it."

Colfer originally intended to write a book that would appeal to boys. "In my experience, I believed that boys are not so interested in character development but I might have been wrong about that," he admits. "With a first book, you always have an element of surprise and that's always vital with a new series. But once you've established that, the story has to become more complicated as it's not so surprising any more. So you've got to make the characters more interesting by putting their lives on the line and occasionally take one or two lives, because otherwise there is no drama."

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The Last Guardian is inspired by the numerous cases of Japanese soldiers who were stranded on remote Pacific islands, completely oblivious to the fact that World War II had ended 10-20 years before.

"I transferred that idea to the fairy people in Irish mythology," says Colfer, who harked back to the fabled Battle of Tailtiu. "The humans took over the country and drove the fairies underground. The fairies left these enchanted warriors behind to guard the gate. The Guardians have no idea there's nothing under it anymore because the spell was only meant to last for six months but they're still there 10,000 years later. They know nothing about modern technology, as they're basically these guys with swords who have popped up into modern human culture.

"I thought they were interesting because they're a threat but they're also quite tragic because they're fighting a war for nothing - although most wars are fought for nothing."

Colfer drew on his childhood love of Irish mythology.

"We're very lucky in Ireland, which I didn't realise until I began to travel and talk to kids from around the world," he says.

"The legends of many countries are often not taught in their schools but if you come to Ireland, everyone knows all about our legendary heroes and our magical stories. Ireland really nurtures its story-tellers so it's a great place to make a living if you're a writer."

Irish migrants have also reinvented their legendary tall tales after settling in faraway lands like the United States.

"People often ask me why America loves magic, mythology and the supernatural so much and I think it has a lot to do with the immigrant condition as a lot of people want to keep that connection to home. Their fathers and grandmothers told them stories that they then passed down, so when they watch stuff like The X-Files and Fringe, it's like being brought back to the mother country in a way."

Colfer also explored expat mentalities in his first adult novel, Plugged, which centres around Irish soldier Daniel McEvoy, who now works as a bouncer in seedy New Jersey clubs.

"Displacement is a very modern condition," he says. "Every Irish person can understand that because we've all either been abroad for a while or have relatives who live overseas. We all know that feeling of isolation that you get living somewhere else and, to me, that was part of Danny's character. He thinks of home through rose - or, in this case, emerald-coloured spectacles - where it's all Danny Boy, U2 and rolling hills, which means he's forgotten why he left in the first place."

Influenced by Burning Chrome author Colin Bateman, Daniel's brutal attempt to avenge the mob murder of his girlfriend is alleviated by Colfer's usual irreverent humour. "I wanted it to be funny but you can only take that so far in a crime book," he says.

"But I'm more comfortable now, so the next one will have more comedy in it. But I'll have to try and keep the dark side in it as well and not make it just pratfalls with custard pies. If you go over the line, it becomes a flat-out comedy book and it won't be taken seriously as a crime novel. Not that I want to be taken seriously but I want it to be a good read."

The Last Guardian is published by Penguin in July; Plugged (Hachette $29.99) is out now.